Method for producing pulp and lignin

ABSTRACT

The invention provides for methods for producing pulp (comprising cellulose) and lignin from lignocellulosic material, such as wood chips. The methods involve acid catalyzed hydrolysis. Lignocellulosic material having a relatively high moisture concentration can be used as the starting material. The lignocellulosic material is impregnated with an acid (preferably nitric acid) and heated. During the heating lignin is depolymerized at relatively low temperatures, and the acid catalyst is distilled off. The acid catalyst can be collected and recycled after impregnation and heating. The lignocellulosic material is then digested in an alkaline solution under heat, dissolving the lignin and allowing the pulp to be removed. Acid is added to the black liquor to precipitate the lignin which is then removed. The resultant amber liquor can be further processed into other ancillary products such as alcohols and/or unicellular proteins.

REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/474,961, filed Jun. 3, 2003, and U.S. ProvisionalApplication No. ______ unassigned, filed Oct. 6, 2003.

TECHNICAL FIELD

[0002] This invention pertains to a method for producing pulp(comprising cellulose) and lignin from lignocellulosic material such aswood chips.

BACKGROUND

[0003] Wood is a composite material comprising cellulose, lignin andhemicellulose.

[0004] Cellulose is the strong, fibrous component which consists of longchains of glucose molecules and which is used to make paper. Lignin is aphenolic polymeric matrix which holds the cellulose fibres together.Hemicellulose is the component comprised of short, branched chains ofglucose and other like molecules.

[0005] In general terms, pulping processes involve converting wood chips(or other suitable vegetative material) into a fibrous form in order toproduce pulp. Pulp refers to cellulose fibres or material containingcellulose fibres which may be used in the production of paper or papercontaining products.

[0006] The two main types of pulping processes in use today aremechanical pulping and chemical pulping though there are other pulpingprocesses, such as chemi-thermal mechanical pulping, which are acombination thereof.

[0007] Mechanical pulping involves the physical separation of individualfibres of the wood by forcing debarked logs and hot water between largerotating steel discs with teeth that tear the wood apart, oralternatively, by pressing logs against grindstones. Generally, pulpsproduced by mechanical pulping are of lower quality than pulps producedaccording to chemical pulping, and thus are used to produce newsprintand cardboard type products.

[0008] On the other hand, chemical pulping involves subjecting wood toheat and chemicals in order to dissolve the lignin and hemicellulosebinding materials, thereby separating the cellulosic fibres. Kraftpulping is an example of a chemical pulping process, and involvesinvolves cooking wood chips in a pressurized vessel known as a digesterin the presence of hot caustic soda (NaOH) and sodium sulfide (Na₂S).

[0009] Kraft pulping is a variation of the soda process where only NaOHis used to break down and remove the lignin. Relative to the sodaprocess, the addition of the sodium sulfide in the Kraft processassisted in dissolving the lignin with less damage to cellulosic fibres.

[0010] In Kraft pulping, the digestion process dissolves the lignin that“glues” the cellulosic fibers to each other in the wood. The resultantsolution containing the dissolved ligneous material is referred to as a“black liquor”. After digestion, the cellulose-containing pulp isseparated from the black liquor and washed. At this stage, the resultantcolored slurry of cellulose fibres is referred to as “brownstock”. Thecolor of the slurry is the result of some residual lignin remainingwithin the slurry with the cellulosic fibres. If desired, a further stepof bleaching the slurry can be carried out to remove some additionalcomponents of the lignin, thereby brightening the pulp. Typically, thepulp is bleached in multiple stages with various combinations of oxygen,hydrogen peroxide and sodium hydroxide. Chelation stages can be utilizedin the bleaching process to remove transition metals in the pulp whichmay otherwise interfere with peroxide bleaching.

[0011] One disadvantage with Kraft pulping is that it is carried outunder extreme conditions which adversely affect yields. Kraft pulpingtypically involves pressures of approximately 120 p.s.i., temperaturesof approximately 160-180° C. and initial pH values over 12 in thedigestion stage. In part, these reaction conditions are required by thefact that reactions carried out in Kraft pulping are oxidationreactions. These harsh conditions result in a loss of some cellulose andthe destruction of much of natural form lignin from the wood chips.

[0012] Lignin typically is not recovered as an end-product in typicalKraft pulping processes. Instead, in Kraft pulping processes the blackliquor is typically condensed in recovery boilers in the process ofrecovering chemicals such as sodium hydroxide for reuse. Solids such aslignin are burned as fuel to run the pulp mill utilizing the Kraftprocess. It would be advantageous to recover as much lignin as possiblein a pulping process since lignin is a valuable component in its ownright, having a wide range of industrial applications such in themanufacture of fertilizers, asphalt emulsifiers, soil stabilizers, woodproducts (such as plywood and particle board), oil well drilling fluidsand dispersing agents for preparing concrete.

SUMMARY OF INVENTION

[0013] In one aspect the invention provides for methods for producingpulp and lignin from lignocellulosic material, the pulp comprisingcellulose. The methods comprise: (a) contacting the lignocellulosicmaterial with an aqueous acid solution to impregnate the lignocellulosicmaterial, the aqueous acid solution comprising from about 10% to about40% by weight of the acid; (b) heating the lignocellulosic material intwo stages, the first heating stage being carried out for a period oftime which is sufficient to depolymerize lignin within thelignocellolosic material without substantially degrading the celluloseor lignin in the ligncellulosic material, the second heating stage beingcarried out at or above the boiling point of the acid to distill off theacid; (c) contacting the lignocellulosic material with an aqueousalkaline solution under heat to solubilize lignin in the alkalinesolution, leaving a black liquor; (d) removing the pulp from the blackliquor; (e) adding sufficient acid to the black liquor to precipitatethe lignin; and (f) removing the lignin from the liquor.

[0014] The acid in step (a) may be nitric acid. The aqueous acidsolution in step (a) may comprise, without limitation, from about 10% toabout 30%, from about 15% to about 30%, from about 20% to about 25%, orfrom about 10% to about 15%, by weight of the acid.

[0015] The lignocellulosic material may be contacted with the aqueousacid solution in step (a) for at least 30 minutes, including withoutlimitation, from about 12 hours to about 24 hours. The lignocellulosicmaterial may be contacted with the aqueous acid solution in step (a)under heat. The lignocellulosic material may be contacted with theaqueous nitric acid solution in step (a) at a temperature from about 50°C. to below the boiling point of the nitric acid.

[0016] The temperature during the second heating stage of step (b) maybe, without limitation, from about 73° C. to below 100° C. or from about90° C. to about 95° C. The temperature during the first heating stage ofstep (b) may be, without limitation, up to about 75° C., from about 50°C. to about 75° C., from about 60° C. to about 70° C., from about 50° C.to about 60° C., or from about 70° C. to about 75° C. The first heatingstage in step (b) may be carried out for at least 15 minutes.

[0017] The aqueous alkaline solution may comprise sodium hydroxide orpotassium hydroxide or a combination of sodium hydroxide and potassiumhydroxide. The aqueous alkaline solution may comprise an amount ofalkali solute which is at least the normal equivalent of the nitric acidin the aqueous acid solution in step (a). The aqueous alkaline solutionmay comprise an amount of alkali solute which is at least the molarequivalent of the nitric acid in the aqueous acid solution in step (a).

[0018] The acid used in step (e) may be sulfuric acid. The amount ofacid added in step (e) may be at least the molar amount of the alkali inthe aqueous alkaline solution in step (c). The amount of acid added instep (e) may be at least the normal amount of the alkali in the aqueousalkaline solution in step (c).

[0019] The black liquor may cooled from the temperature in step (c)before the acid is added in step (e). The temperature of the blackliquor when the acid is added in step (e) may be, without limitation, upto about 75° C., from about 5° C. to about 75° C., from about 5° C. toabout 50° C., from about 25° C. to about 50° C., or from about 50° C. toabout 75° C.

[0020] Any aqueous acid solution not absorbed by the lignocellulosicmaterial in step (a) may be removed prior to heating the lignocellulosicmaterial in step (b). The methods may comprise collecting any aqueousacid solution which is removed prior to step (b) and recycling thecollected aqueous acid solution for use in step (a), and comprisingcollecting any acid which is distilled off in step (b) and recycling thecollected acid for use in step (a).

[0021] The starting moisture content of the lignocellulosic material maybe from about 30% to about 55% by weight of water. The method maycomprise contacting the lignocellulosic material with water before step(a) to increase the moisture content in the lignocellulosic material.The starting lignocellulosic material may comprise, without limitation,wood chips, wood shavings, sawdust, pieces of rye, wheat, hemp andcombinations of two or more of the foregoing. The startinglignocellulosic material may comprise undigested lignocellulosicmaterial previously subjected to the method.

[0022] An amber liquor may be left following the removal of the lignin,and the method may comprise processing the amber liquor after the ligninhas been removed, for example, to produce unicellular proteins oralcohols or both. Any water which is produced as a reaction byproduct inone or more of the steps in the method may be collected and recycled foruse in the method.

[0023] Steps (a) and (c) and the first heating stage in step (b) mayeach carried out at a temperature from about 50° C. to about 75° C. Oneor more of the heating of the impregnated lignocellulosic material instep (b), the contacting the lignocellulosic material with the aqueousalkaline solution in step (c) and the adding the acid in step (e) may becarried out with agitation. The methods may be carried out atatmospheric pressure.

[0024] The pulp removed in step (d) may be washed, pressed, bleached anddried. Similarly, the lignin removed in step (f) may be dried. Anyresidual liquor that is removed from the pulp by pressing may becollected and added to the black liquor prior to adding the acid in step(e). Similarly, any residual liquor which is removed from the ligninduring drying may be collected and added to the liquor after step (f),and wherein the liquor is processed after the lignin has beenprecipitated and removed.

[0025] Enough acid may be added to the solution in step (e) to lower thepH of the solution to an acidic pH.

[0026] Without limitation, step (c) may be carried out at a temperatureup to about 75° C., from about 5° C. to about 75° C., from about 50° C.to about 75° C., from about 20° C. to about 50° C., from about 30° C. toabout 40° C., or from about 40° C. to about 50° C.

[0027] Step (a) may comprise immersing the lignocellulosic material inthe aqueous acid solution in step (a), or spraying the lignocellulosicmaterial with aqueous nitric acid solution.

[0028] In another aspect the invention provides for methods forproducing pulp and lignin from lignocellulosic material, the pulpcomprising cellulose, the methods comprise: (a) contacting thelignocellulosic material with an aqueous nitric acid solution toimpregnate the lignocellulosic material, the aqueous nitric acidsolution comprising from about 10% to about 40% by weight of the nitricacid; (b) heating the impregnated lignocellulosic material in twostages, the first heating stage being carried out a temperature fromabout 50° C. to about 75° C. for a period of time which is sufficient todepolymerize lignin within the lignocellolosic material withoutsubstantially degrading the cellulose or lignin in the ligncellulosicmaterial, the second heating stage being carried out at or above theboiling point of the nitric acid to distill off the nitric acid; (c)contacting the lignocellulosic material with an aqueous alkalinesolution at a temperature from about 50° C. to about 75° C. tosolubilize lignin in the alkaline solution, leaving a black liquor, theaqueous alkaline solution comprising an amount of alkali which is atleast the normal amount of the nitric acid in the aqueous acid solutionin step (a); (d) removing the pulp from the black liquor; (e) coolingthe black liquor and then adding an acid to the black liquor to acidifythe solution to precipitate the lignin; (f) removing the lignin, leavingan amber liquor; and (g) processing the amber liquor to produceunicellular proteins or alcohols or both, wherein any aqueous nitricacid not absorbed by the lignocellulosic material in step (a) is removedand collected following step (a) prior to heating the lignocellulosicmaterial in step (b) and then recycled for use in step (a), and whereinany nitric acid which is distilled off is collected prior to contactingthe lignocellulosic material with the alkaline solution in step (c) andthen recylcled for use in step (a), and wherein the heating of thelignocellulosic material in step (b), the contacting the lignocellulosicmaterial with the aqueous alkaline solution in step (c) and the additionof the acid in step (e) are each carried out with agitation.

[0029] In yet another aspect the invention provides for methods forproducing pulp and lignin comprising the steps of contactinglignocellulosic material with an acid and, after removing any acid notabsorbed by the lignocellulosic material, heating the lignocellulosicmaterial at a temperature up to about 75° C. to effect theacid-catalyzed hydrolytic depolymerization of the lignin in thelignocellulosic material without substantially degrading the celluloseor lignin in the lignocellulosic material, the acid-contacting andheating steps being carried out before the lignocellulosic material isdigested in an alkaline liquor, the pulp being removed following thedigestion of the lignocellulosic material in the alkaline liquor, thelignin being removed after being precipitated out with the addition ofan acid to the black liquor produced following the digestion of thelignocellulosic material in the alkaline liquor, wherein the pulpcomprises cellulose. The acid-contacting step may comprise immersing thelignocellulosic material in an aqueous solution of the acid, and whereinthe acid is nitric acid and the aqueous solution comprises from about10% to about 40% by weight of nitric acid. The lignocellulosic materialmay be heated, after depolymerizing the lignin but before digesting thelignocellulosic material, at a temperature above the boiling point ofthe acid in order to distill off the acid.

[0030] In yet another aspect the invention provides methods for treatinglignocellulosic material comprising: (a) contacting the lignocellulosicmaterial with an aqueous acid solution to impregnate the lignocellulosicmaterial, the aqueous acid solution comprising from about 10% to about40% by weight of the acid; (b) heating the lignocellulosic material intwo stages, the first heating stage being carried out for a period oftime which is sufficient to depolymerize lignin within thelignocellolosic material without substantially degrading the celluloseor lignin in the ligncellulosic material, the second heating stage beingcarried out at or above the boiling point of the acid to distill off theacid, wherein any aqueous acid solution not absorbed by thelignocellulosic material in step (a) is removed prior to heating thelignocellulosic material in step (b); (c) contacting the lignocellulosicmaterial with an aqueous alkaline solution under heat to solubilizelignin in the alkaline solution, leaving a black liquor; and (d)removing the pulp from the black liquor, the pulp comprising cellulose.

[0031] In yet another aspect the invention provides methods forproducing pulp and lignin from lignocellulosic material, the pulpcomprising cellulose, the methods comprising: (a) contacting thelignocellulosic material with an aqueous nitric acid solution toimpregnate the lignocellulosic material, the aqueous acid solutioncomprising from about 10% to about 40% by weight of the nitric acid; (b)heating the lignocellulosic material in two stages, the first heatingstage being carried out for a period of time which is sufficient todepolymerize lignin within the lignocellolosic material withoutsubstantially degrading the cellulose or lignin in the ligncellulosicmaterial, the second heating stage being carried out at or above theboiling point of the acid to distill off the acid, wherein any aqueousacid solution not absorbed by the lignocellulosic material in step (a)is removed prior to heating the lignocellulosic material in step (b);(c) contacting the lignocellulosic material with an aqueous alkalinesolution under heat to solubilize lignin in the alkaline solution,leaving a black liquor; (d) removing the pulp from the black liquor; (e)adding sufficient acid to the black liquor to precipitate the lignin;and (f) removing the lignin from the liquor.

[0032] Further aspects of the invention and features of specificembodiments of the invention are described below.

BRIEF DESCRIPTION OF DRAWINGS

[0033]FIG. 1 is an illustration of the mechanism of acid-catalyzedhydrolysis of an ester; and

[0034]FIG. 2 is a schematic illustration of a method according to thepresent invention.

DESCRIPTION

[0035] Throughout the following description specific details are setforth in order to provide a more thorough understanding of theinvention. However, the invention may be practiced without theseparticulars. In other instances, well known elements have not been shownor described in detail to avoid unnecessarily obscuring the presentinvention. Accordingly, the specification and drawings are to beregarded in an illustrative, rather than a restrictive, sense.

[0036] Most existing pulping processes involve high pressure and/or hightemperature conditions or concentrated chemicals. Not only do thesefactors increase costs, but they also reduce the yield and purity ofcellulose-containing pulp and lignin end-products since the celluloseand lignin can be degraded or destroyed during the pulping processes.The present invention addresses these deficiencies by utilizing the acidcatalyzed hydrolysis of lignocellulosic material in methods forproducing pulp and lignin from lignocellulosic material. The hydrolysisreaction allows the controlled de-polymerization of lignin within thestarting lignocellulosic material, utilizes much lower concentrations ofpulping chemicals and lower temperatures than existing Kraft pulpingprocesses and may be carried out at atmospheric pressure. These modestreaction conditions not only result in reduced costs relative toexisting Kraft pulping processes, but also increase the yield and purityof the cellulose-containing pulp and lignin since there is lessdegradation and and destruction of these components during the pulpingprocess relative to existing Kraft pulping processes.

[0037] The invention includes methods involving contactinglignocellulosic material with an acid, preferably nitric acid, in orderto impregnate the lignocellulosic material and then heating thelignocellulosic material to de-polymerize the lignin within thelignocellulosic material prior to digesting the lignocellulosic materialin an alkaline solution to solubilize lignin in the alkaline solution,leaving a black liquor (comprising the solubilized lignin) and pulp. Thepulp is then removed and processed (e.g. pressing, washing, bleaching,drying). The solubilized lignin is precipitated out of the black liquorwith the addition of an acid and then removed for processing. Theremaining solution can be further processed to produce other usefulby-products such as unicellular proteins, alcohols or both.

[0038] As used herein, “lignocellulosic material” refers to any materialwhich contains cellulose and lignin and, without limitation, includespieces or particles (for example wood chips, wood shavings, sawdust)from any type of tree (for example pine, oak, maple, fir, spruce) andother vegetative material (for example rye, wheat hemp) and combinationsthereof.

[0039] As used herein, “impregnation” refers to the absorption of animpregnating material by lignocellulosic material. For example,impregnating wood chips can be accomplished by immersing or soaking thewood chips in a sufficient volume of the impregnating material (e.g.nitric acid solution) to saturate all or part of the wood chips. Othermethods such as spraying could also be used to accomplish theimpregnation.

[0040] The separation of lignin and cellulose in their natural forms hasproven difficult in practice. Many existing pulping processes areinefficient, with valuable cellulose and lignin being degraded, modifiedor destroyed during pulping, or less than optimal separation betweencellulose and lignin being achieved in the pulping process. Thedifficulty in separating lignin and cellulose arises from the inherentinstability and fragile nature of pure lignin. Lignins are formed byremoval of water from sugars to create aromatic structures and thesereactions are irreversible. There are many possible monomers of lignin,and the types and proportions depend on the source in nature. Sometypical monomers of lignin are shown below:

[0041] The hydroxyl (—OH) groups, either the hydroxyl groups on thechains or the hydroxyl groups on the aromatic rings, can react with eachother or with the aldehyde or ketone groups. A ether linkage is formedwhen a hydroxyl group reacts with another hydroxyl group, a hemiacetalis formed when a hydroxyl group reacts with an aldehyde and a ketal isformed when a hydroxyl group reacts with a ketone. An early stage in thecondensation of various monomers to form lignin is shown below, whereinthere are several groups shown that can react further:

[0042] Some monomers will simply extend the polymer while others willestablish cross-linking. The shaded monomer has three of its functionalgroups linked to other monomers, so it is starting a branch orcross-link. Lignin molecules are three-dimensional and heavilycross-linked. A typical lignin molecule can have a molecular weight ofabout 15,000 amu. Lignin molecules are susceptible to harsh chemicalconcentrations and temperature exposure. A lignin polymeric matrix maybe broken down at temperatures as low as 100° C. Lignin is unstable,light sensitive, and breaks down into acid compound as it ages, and itspresence in paper items contributes to their degradation over time.

[0043] As mentioned above, the present invention utilizes theacid-catalyzed hyrolysis of lignocellulosic material. The lignin withinlignocellulosic material includes ester functional groups. FIG. 1illustrates the mechanism of the acid-catalyzed hydrolysis of an ester.The rate of ester hydrolysis is very slow because their leaving groupsare basic. The rate can be increased by the presence of a catalyst, suchas an acid. In such an acid catalyzed reaction all organic reactants,intermediates, and products are positively charged. The following stepstake place within the hydrolysis of the ester:

[0044] (A) An acid/base reaction is illustrated. Since there is only aweak nucleophile and a poor electrophile the ester needs to beactivated. The acid protonates the ester carbonyl, making it moreelectrophilic.

[0045] (B) The oxygen atom in the water molecule functions as thenucleophile, attacking the electrophilic carbon atom in the C=0.Electrons move towards the oxonium ion, thereby creating a tetrahedralintermediate.

[0046] (C) Another acid/base reaction is illustrated. The oxygen atom inthe water molecule is deprotonated to neutralize the charge.

[0047] (D) Another acid/base reaction is illustrated. The —OCH₃ mustleave, but first must be converted into a more stable group by beingprotonated.

[0048] (E) The electrons from the adjacent oxygen atom assist theformation of the leaving group, a neutral methanol molecule CH₃OH.

[0049] (F) Another acid/base reaction. The oxonium ion is deprotonated,which exposes the carbonyl C═O carboxylic acid product and regeneratesthe acid catalyst.

[0050] The carboxylic acid product obtained in (F) is a de-polymerizedform of lignin.

[0051] Because water (H₂O) and methanol (CH₃OH) have approximately thesame basicity, it will be equally easy for the tetrahedral intermediatesto collapse to reform the ester as it would be for the formation of thecarboxylic acid. Hence, at equilibrium both the ester and the carboxylicacid will be obtained. However, an excess of water will shift theequilibrium towards the formation of the carboxylic acid product,de-polymerized lignin. Thus, methods of the present invention willrender higher yields with the addition of water and, for this reason, itis possible to use lignocellulosic material which has a relatively highmoisture content (e.g. “green” wood chips) or which has been previouslycontacted with water in order to increase the moisture content prior toimpregnating the lignocellulosic material with the acid.

[0052] The ability to use lignocellulosic materials with a relativelyhigh moisture content in the present invention is an advantage comparedto existing Kraft type pulping processes. In many existing processes itis necessary to dry wood chips prior to pulping in order to remove orreduce moisture to meet stringent requirements. In fact, chips arepreferred in many existing processes on a BDU (bone dry unit) basis.Requiring a drying step adds expense to such processes. The presentinvention overcomes this disadvantage since “wet” starting materials canbe used, thus removing the need to dry the wood chips or other startingmaterials. Moreover, as explained above in relation to FIG. 1, sinceexcess water drives the depolymerization reaction to favor the formationof a carboxylic acid instead of collapsing to reform an ester,additional moisture in the chips improves yields in methods according tothis invention.

[0053] Following depolymerization of lignin, the lignocellulosicmaterial is digested in the presence of an alkali such as sodiumhydroxide (NaOH) and/or potassium hydroxide (KOH). The positive cations(K⁺ or Na⁺ or another suitable ion) then attacks the exposed —OH on thelignin product (carboxylic acid) bumping out the hydrogen cation (H⁺).The hydrogen cation then bonds with the negatively charged hydroxyl ionto form water. The lignin is then water-soluble and drops into solution,thereby separating the dissolved lignin from the cellulose-containingpulp. Due to more modest temperature, pressure and reactantconcentrations involved, the lignin and cellulose produced are subjectto less degradation than products obtained through a Kraft type process,thereby providing increased yield and purity.

[0054]FIG. 2 is a schematic illustration of a method 10 according to thepresent invention. Lignocellulosic material, such as wood chips (notshown), is used as the starting raw material in an impregnating tank 12.The wood chips are prepared by chipping wood into a convenient size, forexample, chips of roughly 0.5-2.0 inches in diameter. This can be donewith a standard mill. Other sizes and shapes of lignocellulosic materialcan be used. Given the hydrolysis involved in the method, preferably“green” wood chips are used, or in other words, chips having a startingmoisture content of approximately 30-55%, or even higher, by weight ofwater. A pre-treatment stage could be utilized wherein the wood chipswere immersed or otherwise contacted with water prior to being placed inthe impregnating tank in order to increase the moisture content withinthe chips up to or beyond 30-55% by weight of water, though this step isnot necessary to produce lignin, pulp and other ancillary productsaccording to this invention. Impregnating bone dry chips with an acidsolution could be done, but this will result in higher consumption ratesof the acid impregnant, thereby increasing costs.

[0055] Wood chips are loaded into impregnating tank 12 where they arecontacted with an acid in order to impregnate the wood chips with theacid. The acid is preferably provided within an aqueous solution, and insuch a case, the acid impregnates the wood chips as part of thesolution. That is, the wood chips absorb both acid and water. Accordingto one aspect of the invention, the aqueous solution comprises fromabout 10% to about 40% by weight of the acid. The invention alsoincludes utilizing other, narrower acid concentration ranges within thisrange. For example, the aqueous acid solution may comprise from about15% to about 30%, from about 10% to about 25%, from about 10% to about15%, or from about 20% to about 25%, by weight of the acid.

[0056] The aqueous acid solution is provided to impregnating tank 12from acid solution tank 14 in a sufficient volume to immerse the woodchips. The acid used is preferably nitric acid, but those skilled in theart will appreciate that other acids could instead be used within thescope of the present invention. Nitric acid is used herein forillustration purposes.

[0057] The wood chips are impregnated by allowing them to soak in theaqueous nitric acid solution in the impregnating tank 12. Theimpregnation time may last for as little as 30 minutes or less and mayextend for any length of time before the degradation of the wood chipsbegins to occur. For example, impregnation may last from about 2 hoursand 45 minutes to about 48 hours. It is contemplated that theimpregnation time may fall within a narrower time ranges such as, forexample, from about 12 to about 24 hours and ranges within this range.

[0058] Heating the chips during the impregnation step may decrease theimpregnation time. For example, impregnation of the wood chips couldoccur from about 30 minutes or less to about 13 hours with the additionof heat, or up to 48 hours or longer, without the addition of heat.Heating during impregnation can be achieved by any number of well knowntechniques that could be used to heat the wood chips within impregnatingtank 12. For example, the exterior of impregnating tank could be heatedwith steam to heat the tank 12. After sufficient time has passed tocomplete impregnation, any excess nitric acid solution is removed fromthe wood chips, by drainage for example, and passed to spent solutiontank 16. Any collected excess nitric acid solution may then be recycledfor use in the impregnating tank 12. Preferably any excess nitric acidsolution is filtered and purified using conventional techniques beforebeing reconstituted back up to strength. Filtration and/or purificationcould be accomplished by passing any spent nitric acid solution fromspent acid solution tank 16 to a conventional filtration and/orpurification apparatus (not shown) before the solution is passed to acidsolution tank 14. However, it may not be necessary to filter and purifythe excess nitric acid solution if there are not significant amounts ofcontaminants present.

[0059] The impregnated wood chips are then passed to a heating tank 18.This transfer can be accomplished by gravity feed, if impregnating tank12 is located above the heating tank 18, or by a suitable transfermechanism if not. The impregnated chips are heated within heating tank18. The exterior of heating tank 18 is heated with steam produced bysteam boiler 20, thereby heating the chips within heating tank 18. Manyother known apparatus or techniques could alternatively be used to heatheating tank 18. Heating of the chips in heating tank 18 may be carriedout in two heating stages; the first heating stage to depolymerize thelignin within the wood chips without substantially degrading thecellulose or lignin, and the second heating stage to distill off theacid following depolymerization of the lignin. The distilled off nitricacid is recycled for use in the impregnation stage. In particular, thedistilled off nitric acid is collected and passed to a condenser 22 andthen to spent solution tank 16 to be reconstituted to a desired strengthand then passed back to nitric acid tank 14 for eventual use inimpregnating tank 12.

[0060] The nitric acid attacks the cross-links or ester groups of thelignin polymeric matrix during the first heating stage. The firstheating stage is carried out for a period of time which is sufficient todepolymerize the lignin within the wood chips without substantiallydegrading the cellulose or lignin. When nitric acid is used as theimpregnating acid, preferably the first heating stage is carried out ata temperature up to about 75° C., but above the freezing point of theaqueous acid solution It is within the scope of the invention for thefirst heating stage to be carried out within a narrower temperaturerange, for example from about 50° C. to about 75° C., from about 50° C.to about 60° C., from about 70° C. to about 75° C., or from about 60° C.to about 70° C., when nitric acid is used as the impregnating acid. Itis desirable to maintain these more modest temperatures (relative toKraft pulping temperatures) in order to prevent the unwanted degradationand destruction of the lignin and cellulose within the wood chips, thusproviding improved product yield and purity.

[0061] Preferably heating tank 18 is fitted with an agitator 24 toprovide agitation during the heating of chips within heating tank 18.Aggressive agitation by agitator 24 helps to both bring about ahomogeneous temperature within heating tank 18 and to begin breakingapart the fibers in the wood chips.

[0062] During the second heating stage, the temperature is then broughtabove the boiling point of the acid for a sufficient time (for example,30 minutes) to distill off the nitric acid. It is important to note thatthe temperature of the boiling point of the acid in this stage maydepart slightly from the temperature that one would expect the acid toboil. This discrepancy results from various factors including theinteraction of the acid with the wood, the moisture content within thewood, and, for nitric acid, the formation of azeotropes with water. Forexample, whereas some scientific literature establishes the boilingpoint of nitric acid to be 83° C., the inventor has observed boiling fornitric acid with wood chips as low as 73° C. Thus, it is preferred forthe temperature of the second heating stage within heating tank 18 be atleast about 73° C. where nitric acid is utilized. Similarly, it is alsopreferred to maintain the temperature during the second heating stagebelow the boiling point of water, 100° C., to avoid the accidentaldistilling off of wood chip moisture, which would initiate the burningof the wood chips. Thus, according to one aspect of the invention, thesecond heating stage could be carried out from about 73° C. to below100° C. Again, it is within the scope of the invention to utilizenarrower ranges, for example from about 90° C. to about 95° C., withinthis larger range.

[0063] The amount of time for the first and second heating stages willvary depending upon a number of factors, including the particulartemperatures involved. For example, the first heating stage may becarried out for as little as 15 minutes, and could be carried out for alonger period of time.

[0064] Following the heating in the heating tank 18, the wood chips arethen transferred to a digester 26. There the wood chips are digested bybeing contacted with a caustic solution, or in other words, an aqueousalkaline solution wherein the solute is an alkali such as sodiumhydroxide, potassium hydroxide, or a combination of these.

[0065] Those skilled in the art will appreciate that other bases havingsimilar chemical properties as these bases could be used. The wood chipsare contacted with the aqueous alkaline solution under heat tosolubilize the lignin in the solution. The resultant solution is darkerin color due to the solubilized lignin and is thus referred to as blackliquor. This digestion step thereby separates the lignin and the blackliquor from the pulp. The alkaline solution is provided to digester 26from caustic solution tank 28. The concentration of the alkalinesolution is such to just make the lignin water soluble. This isaccomplished by the positive cations (K⁺ or Na⁺ or another suitable ion)attaching to the exposed cleavage sites on the lignin monomers and thelignin then dissolving into the water.

[0066] The temperature during the digestion stage is preferably up toabout 75° C., but above the freezing point of the aqueous alkalinesolution. It is however within the scope of the invention to increasethe temperature above 75° C. up to the boiling point of the alkalinesolution. For example, where the alkaline solution comprises an aqueoussodium hydroxide solution, it is possible for the temperature during thedigestion step to be from about 75° C. to about 95° C. It is also withinthe scope of the invention for the digestion stage to be carried outwithin a narrower temperature range, for example, from about 5° C. toabout 75° C., from about 50° C. to about 75° C., from about 20° C. toabout 50° C., from about 30° C. to about 40° C., or from about 40° C. toabout 50° C. Heat is provided to digester 26 by heater 30, which may beany conventional apparatus or arrangement for providing heat to thedigester 26.

[0067] Digester 26 may be fitted with an agitator 32 to provideagitation during the digestion of the chips. Aagitation helps to bringabout a homogeneous temperature within the digester 26 and to break upthe wood chips. Agitation also increases the rate that the cation (e.g.K⁺, Na⁺, etc.) binds to the exposed lignin cleavage sites thusminimizing any possible caustic action on the cellulose.

[0068] For cost and yield reasons, it is desirable to use as littlealkali as possible within the caustic solution in the digestion of thewood chips. In one aspect of the invention, the alkaline solution mayinclude an amount of alkali which is at least the normal equivalent ofthe nitric acid in the solution used in the impregnation stage. Addingexcess alkali will increase costs by requiring more acid to be added ata later step. Further, the alkali concentration in the aqueous alkalinesolution should be maintained below 17.5% by weight of the solutionsince beta and gamma cellulose dissolve at this level.

[0069] Following digestion, the resultant pulp (containing cellulose)and black liquor (containing solubilized lignin) is transferred to pulptank 34. The black liquor is removed, for example by drainage, andtransferred to black liquor tank 36. The cellulose-containing pulp isthen processed according to the user's needs.

[0070] Typically, processing would involve washing, pressing, bleachingand drying the pulp following the separation from the black liquor. Forexample, in FIG. 2, the pulp would be pressed in a pulp press 38 toremove any residual black liquor (which would also be transferred toblack liquor tank 36), and then bleached and dried with bleachingapparatus 40 and dryer 42. Bleaching apparatus 40 and dryer 42 could beany conventional known apparatuses for these purposes.

[0071] The lignin is precipitated out of the black liquor in blackliquor tank 36 after the addition of an acid from acid solution tank 44.Preferably the acid added is a mineral acid, such as sulfuric acid. Theacid added to the black liquor strips off the cation from the causticsolute, thereby precipitating out the lignin and a caustic salt (e.g.where sulfuric acid is used, the salts Na₂SO₄, K₂SO₄, or other similarsalts depending upon the alkali solute used in digestion, would result).Addition of the acid to the black liquor lowers the pH of the liquoruntil it is just slightly acidic. The acid may be added to the blackliquor within an aqueous solution. For cost and yield reasons, it isadvantageous if the acid solution is as dilute a solution as possible.Black liquor tank 36 may be fitted with an agitator 46 to provideagitation during the addition of the acid from tank 44. The contents ofblack liquor tank 36 may be cooled, for example by cooler 48, in orderto faciliate the precipitation of lignin. Cooler 48 can be any knownapparatus for cooling purposes. The black liquor may be cooled from thedigestion temperature before the addition of the acid. Preferably thetemperature is up to about 75° C., but above the freezing point of theblack liquor. Other narrower temperature ranges could be used at thisstep such as from about 5° C. to about 75° C., 5° C. to about 75° C.,from about 50° C. to about 75° C., or from about 25° C. to about 50° C.

[0072] The precipitated lignin can then be removed from the resultantamber liquor (which is lighter in color than the black liquor due to theremoval of the lignin) and processed using any conventional technique.For example, the resultant amber liquor and lignin slurry may be passedfrom black liquor tank 36 through a lignin filter 50, with the ligninthen being transferred to a lignin dryer 52 and dried at temperaturesthat will not degrade the lignin. The resultant lignin contains naturalform lignin.

[0073] The filtrate passing through lignin filter 50 is the amberliquor, which is then transferred to an amber liquor tank 54. Anyresidual amber liquor removed during the drying of lignin in lignindryer 52 is also transferred to amber liquor tank 54, as illustrated inFIG. 2. Amber liquor is an aqueous solution containing starches, sugars,and other minerals and compounds found in plants that are not celluloseor lignin. In amber liquor tank 54, the amber liquor can be processed toyield various other products of value including alcohols, unicellularproteins or both. For example, the amber liquor could be used as astarting culture for various bacteria to produce various products, suchas animal feeds and alcohols. Amber liquor tank 54 may be fitted with anagitator 56 to provide agitation if desired. Any proteins producedduring the processing of the amber liquor may be removed by filtration(via protein filter 58) and dried (via protein dryer 60). Suitableapparatus modifications and/or additions could be made to accommodatealcohol processing and/or the production of other desired end products.The final filtrate can be recovered and treated in water treater 62 toremove any contaminants, thereby allowing the recycling of water withinvarious steps in the method, for example in reuse in washing in pulptank 34, preparation of the alkaline solution in caustic solution tank28 and in steam boiler 20. Any recovered water could also be used insteam boiler 20 or in the preparation of the aqueous acid solutions inacid solution tank 14 and/or acid solution tank 44.

[0074] Without limitation, the present invention provides the followingbenefits:

[0075] 1) Wet starting materials can be used. Since water is essentialto the hydrolysis of the lignocellulosic material, this removes the needto dry the starting materials prior to the pulping process, thusresulting in lower costs relative to certain existing pulping methods.

[0076] 2) The method of the present invention does not require addedpressure, but may be carried out at atmospheric pressure. This reducescosts relative to certain existing pulping methods, as does the factthat the method of the present invention does not require the additionof heat at temperatures approaching those used in typical Kraft typepulping processes.

[0077] 3) Weak concentrations of strong acids and strong bases may beused, thereby minimizing raw material costs and degradation of finalproducts.

[0078] 4) The acid catalyst can be recovered and recycled for reuse,allowing improved cost efficiency. These factors also allow a closedsystem, minimize pollution, and therefore environmental impact. Verylittle pollution is caused by methods according to this invention.

[0079] 5) Only small amounts of chemicals are needed to bring back tostrength each recovered chemical before being reused in the method.Moreover, unlike recovery stages in certain Kraft type processes,external energy (and the resultant expense) is not required during therecovery of chemicals in methods according to the present invention.

[0080] 6) Water used in the method may be recovered in saleablebyproducts (such as alcohols and animal feeds) treated and reused orvented as steam, with suitable apparatus and process modifications. Thevented steam could be used in providing energy for the method, therebyeliminating even this small loss of water.

[0081] 7) The method is flexible in terms of starting materials. Theability to process a wide variety of lignocellulosic material withoutretooling any apparatus or changing the methods involved givesflexibility in pulp production. Currently, mills are typically designedto produce specific pulp types and utilize specific wood species as rawmaterials. Furthermore, most Kraft mills require chips meeting stringentquality specifications to remain economically viable. The present methodmay utilize not only any number of differently sized chips, but alsosawdust, and also chips that would be considered “green” and unusable bycurrent pulp mill standards. The flexibility provided by methods of thisinvention eliminates or simplifies the need for chip mills designed toproduce chips of stringent standards, thereby offering the potential forlower operating and capital costs.

[0082] 8) The yield of alpha cellulose is high using the method, whilethe method also allows for a high yield of lignin, which is a valuablecomponent itself.

[0083] 9) Aside from cellulose and lignin, other useful and potentiallyvaluable by-products may be recovered using methods of this invention.For example, the amber liquor is suitable for fermentation ofunicellular protein following precipitation and removal of lignin. Theprotein can be used in animal feeds or for research purposes.

[0084] 10) Methods of the invention may be applied to different sizedand configured apparatuses, thereby improving the flexibility of use.

[0085] 11) Mills utilizing a method of the present invention will behighly efficient, with lower operating costs than mills using typicalKraft pulping processes.

[0086] The following examples are presented by way of illustration andnot by way of limitation.

EXAMPLE 1

[0087] This example comprises 19 trials which were run to illustrate theyields of pulp and lignin obtained using methods according to thisinvention, and also to illustrate the recovery of the acid used in theimpregnation stage.

[0088] The starting lignocellulosic material in Trials 1-18 comprised200 grams of a mixture of hardwood and softwood wood chips obtained froma sawmill and chip mill operation in Kelowna, British Columbia, Canada.600 grams of wood chips were used as the starting material in Trial 19.All of the different species of trees that the chips originated fromwere not ascertained, but at least some of the chips came from PonderosaPine, Douglas Fir, Maple, Oak and Spruce.

[0089] The moisture content of the wood chips was between 35-50% waterby weight.

[0090] The moisture content was calculated by drying separate samples ofthe wood chips (which were not subjected to the steps of the method) andmeasuring the weight difference in the wood chips following the dryingstep.

[0091] The acid used in the impregnating step was nitric acid (HNO₃).Nitric acid solutions were prepared by diluting an amount (chosendepending upon the strength of solution desired) of 70% (w/w) nitricacid with distilled water. In each trial, 1 L of an aqueous nitric acidsolution was used in the impregnating step, with the exception thatTrials 17 and 18 each used 750 mL, and Trial 19 used 2250 mL. A 2500 mLbeaker sealed with laboratory film was used as the impregnating tank.

[0092] Following the impregnation step, excess nitric acid solution wasremoved from the wood chips before they were heated in the heating step.The heating step was carried out in a 2000 ml round bottom triple neckboiling flask, which was fitted with a distillation setup so that nitricacid distilled out in the heating step would be collected.

[0093] In order to calculate the efficiency of the method in recoveringthe nitric acid, samples from the excess acid solution collectedfollowing the impregnating step and from the distilled nitric acid whichwas collected during the heating step were each titrated using a 10%(w/w) % solution that was prepared by dissolving 100 g NaOH in distilledwater and topped up in a 1 L volumetric flask. Titration of the excessnitric acid solution collected from after the impregnation steppermitted the calculation of the number of moles of nitric acidrecovered after the impregnating step, which in turn permitted thecalculation of the number of moles of nitric acid which were absorbed bythe wood chips, since the moles of nitric acid in the initial 1 Lsolution was known. After titrating the nitric acid that was distilledout and collected from the heating step, it was possible to calculatethe total moles of nitric acid recovered in the method. By comparingthis number to the amount of moles in a particular starting nitric acidsolution allowed the calculation of the percentage of starting nitricacid which was recovered by the method. Also, it was found that redfuming nitric acid (RFNA) and nitrogen dioxide (NO₂) formed during theheating step and these vapors were lost to the atmosphere. These vaporscould have been retained with better equipment, and if that were thecase, the amount of nitric acid actually recovered would have greatlyincreased. To account for this, the amount of mass lost from the heatingtank was determined by weighing the tank before and after the heatingstage, with the difference which was lost as RFNA and NO₂. On theassumption that this amount could be recoverable as HNO₃ withappropriate equipment modifications, it is then possible to calculatewhat the total moles of HNO₃ which would be recoverable using the methodby adding the amount actually recovered with the amount lost as RFNA andNO₂, and expressing this as a percentage of the starting moles of HNO₃.This is shown in Tables 1-5 for Trials 1-19. All titrations used 2 dropsphenolphthalein as an indicator.

[0094] Following the heating step, the chips were digested in a roundstainless steel tank with a 9.283 liter capacity (22.86 cm diameter,11.43 cm height). The alkaline solution used in digestion was preparedby diluting an amount of 2.5 molar sodium hydroxide (NaOH) solution, 2.5molar potassium hydroxide (KOH) solution, or a combination thereof, withdistilled water. Various volumes of the initial 2.5 molar solutions wereadded to make the final alkaline solution used in digestion and variousvolumes of the final alkaline solution prepared were used for thevarious trials. The moles of each alkali solute(s) and the total volumeof the aqueous alkaline solution prepared for Trials 1-19 are found inTables 1-5.

[0095] In each of Trials 1-19 it took about 10 minutes or less to digestapproximately 95% or more of the wood chips. In certain trials, aportion of the chips remained undigested, and the amounts are indicatedin Tables 1-5 for such trials, as are the total digestion time for eachtrial.

[0096] Following the digestion step, the black liquor (containingsolubilized lignin) was vaccum filtered in a buchner funnel withoutfilter paper to remove the pulp, which was washed with distilled waterand then air-dried at room temperature and weighed. The black liquor wastransferred to a 2500 mL beaker, where the lignin was precipitated withthe addition of a sulfuric acid (H₂SO₄) solution. The sulfuric acidsolution was prepared by diluting an volume (depending upon theconcentration sought) of pure sulfuric acid in a 1 L volumetric flaskwith distilled water. In most cases, the temperature of the black liquorwas allowed to cool from the digestion step for the lignin precipitationstep. The various temperatures are shown in Tables 1-5.

[0097] The lignin was vacuum filtered in a buchner funnel without filterpaper and thus removed from the amber liquor. The lignin was air-driedat room temperature and weighed. The lignin was not however dried andweighed for Trials 17 and 18. In these trials, the lignin didprecipitate and was, by visual inspection, found to have a similarrelative amount and appearance as the lignin produced in Trials 1-16.

[0098] As set out below, heat was applied during the heating anddigestion steps and in five trials, during the impregnation step. Whereso indicated, the heat was supplied in various steps by using a heatingmantle made by Glas-Col Apparatus Co., Cat 10 0410, having 465 totalwatts and 110 volts. The mantle controller was a Powerstat variableautotransformer, Type 3PN116B, made by The Superior Electric Co. ofBristol, Conn. Agitation was provided during the digestion step by acold steel impeller which was 8 inches in diameter with {fraction (1/2)}inch separation between 45° offset blades. A 18 volt DEWALT™ XRPcordless drill was used to power the agitator. Agitation was providedduring the lignin precipitation step by manual stirring using a glassstir rod for approximately 30-45 seconds in each trial.

[0099] The conditions and results of Trials 1-19 are found in Tables1-5. TABLE 1 Trial 1 Trial 2 Trial 3 Trial 4 IMPREGNATION STEP % HNO₃(w/w) 15 24.15 25 30 Impregnating Time (hours) 24 48 48 13 HEATING STEPTime to reach 50° C. (min.) 55 41 40 30 Time between 50-75° C. (min.) 7691 30 75 Time above 75° C. (min.) 59 19 35 32 Minimum HNO₃ DistillationTemp. (° C.) 85 81 81 73 Distillation time (min.) 59 19 35 32 DIGESTIONSTEP Alkaline Solution moles NaOH 0.425 0.625 0 0.435 moles KOH 0 0.1250.625 0.19 Vol. of Alkaline Solution (mL) 2670 3300 3000 3000 DigestionTemp. (° C.) 60 63 50 70 Digestion Time (min.) 30 50 40 25 Amount ofUndigested Chips (g) 0 0 5.4 0 LIGNIN PRECIPITATION STEP % H₂SO₄Solution added to black liquor 20 10 10 20 (v/v) Vol. of Acid Sol'nadded to black liquor 35 140 140 75 (mL) Temp. during Addition of Acid(° C.) 25 45 45 25 RESULTS Pulp recovered (g) 92.1 104.6 99.6 94.5 Pulprecovered (%) 46.05 52.3 49.8 47.25 Lignin recovered (g) 41.4 42.1 39.843.1 Lignin recovered (%) 20.7 21.05 19.9 21.55 HNO₃ recoverable (%)98.44 93.2 91.7 77.7

[0100] TABLE 2 Trial 5 Trial 6 Trial 7 Trial 8 IMPREGNATION STEP % HNO₃(w/w) 15 15 11.5 15 Impregnating Time (hours) 24 21 48 13, at 75-80° C.HEATING STEP Time to reach 50° C. (min.) 15 19 31 35 Time between 50-75°C. (min.) 20 90 65 25 Time above 75° C. (min.) 45 12 15 20 Minimum HNO₃Distillation Temp. 83 81 83 83 (° C.) Distillation time (min.) 45 12 1520 DIGESTION STEP Alkaline Solution moles NaOH 0.55 0 0.25 0.3125 molesKOH 0 0.625 0.3 0.3 Vol. of Alkaline Solution (mL) 3220 3250 3220 3245Digestion Temp. (° C.) 50 50 60 70 Digestion Time (min.) 30 60 15 30Amount of Undigested Chips (g) 7 8.6 14.2 7 LIGNIN PRECIPITATION STEP %H₂SO₄ Solution added to black 10 10 10 10 liquor (v/v) Vol. of AcidSol'n added to black 90 132 150 125 liquor (mL) Temp. during Addition ofAcid (° C.) 40 32 34 61 RESULTS Pulp recovered (g) 92.2 90.1 88.6 101.4Pulp recovered (%) 46.1 45.1 44.3 50.7 Lignin recovered (g) 37.9 39.138.3 42.2 Lignin recovered (%) 19 19.6 19.2 21.1 HNO₃ recoverable (%)93.4 92.1 93.4 88.1

[0101] TABLE 3 Trial 9 Trial 10 Trial 11 Trial 12 IMPREGNATION STEP %HNO₃ (w/w) 15 20 15 24.15 Impregnating Time (hours) 17 at 2:45 at 31 3075-80° C. 75° C. HEATING STEP Time to reach 50° C. (min.) 35 19 37 41Time between 50-75° C. (min.) 24 85 32 105 Time above 75° C. (min.) 2212 114 30 Minimum HNO₃ Distillation Temp. (° C.) 83 75 80 83Distillation time (min.) 22 12 20 30 DIGESTION STEP Alkaline Solutionmoles NaOH 0.625 0.625 0 0.3125 moles KOH 0 0 0.625 0.3125 Vol. ofAlkaline Solution (mL) 3000 3000 2500 2250 Digestion Temp. (° C.) 50-6370 55 60 Digestion Time (min.) 30 15 15 20 Amount of Undigested Chips(g) 0 20.3 24.4 0 LIGNIN PRECIPITATION STEP % H₂SO₄ Solution added toblack 10 10 20 10 liquor (v/v) Vol. of Acid Sol'n added (mL) 150 150 65125 Temp. during Addition of Acid (° C.) 29 38 27 31 RESULTS Pulprecovered (g) 85.6 87.1 77.1 103.5 Pulp recovered (%) 42.8 43.6 38.651.75 Lignin recovered (g) 38.9 43.9 33.9 44.1 Lignin recovered (%) 19.522 17 22.05 HNO₃ recoverable (%) 93.4 92.61 93.8 65.05

[0102] TABLE 4 Trial 13 Trial 14 Trial 15 Trial 16 IMPREGNATION STEP %HNO₃ (w/w) 20 30 15 15 Impregnating Time (hours) 13 15 12 14 HEATINGSTEP Time to reach 50° C. (min.) 19 23 14 6 Time between 50-75° C.(min.) 95 55 48 57 Time above 75° C. (min.) 15 20 35 18 Minimum HNO₃Distillation Temp. (° C.) 81 80 81 79 Distillation time (min.) 15 20 4525 DIGESTION STEP Alkaline Solution moles NaOH 0.625 0 0.5 0.5 moles KOH0 0.625 0 0 Vol. of Alkaline Solution (mL) 2500 2750 2700 2700 DigestionTemp. (° C.) 55 61 73 41 Digestion Time (min.) 20 25 10 13 Amount ofUndigested Chips (g) 4.9 11.6 28.7 102.3 LIGNIN PRECIPITATION STEP %H₂SO₄ Solution added to black liquor (v/v) 10 10 20 20 Vol. of AcidSol'n added to black liquor 150 150 110 130 (mL) Temp. during Additionof Acid (° C.) 26 36 18 5 RESULTS Pulp recovered (g) 96.4 87 71.2 35.2Pulp recovered (%) 48.2 43.5 35.6 17.6 Lignin recovered (g) 47 38.6 27.217.8 Lignin recovered (%) 23.5 19.3 13.6 8.9 HNO₃ recoverable (%) 91.190.6 83.7 84.1

[0103] TABLE 5 Trial 17 Trial 18 Trial 19 IMPREGNATION STEP % HNO₃ (w/w)15 15 15 Impregnating Time (hours) 1:25 at 50-55° C. 2:10 at 50-55° C.12 HEATING STEP Time to reach 50° C. (min.) 12 10 20 Time between 50-75°C. (min.) 60 (held between 55 (held between 35 70-75° C.) 60-70° C.)Time above 75° C. (min.) 11 22 27 Minimum HNO₃ Distillation 83 82 83Temp. (° C.) Distillation time (min.) 11 15 32 DIGESTION STEP AlkalineSolution moles NaOH 0.5 0.475 1.2625 moles KOH 0 0 0 Vol. of AlkalineSolution 2500 2690 3050 (mL) Digestion Temp. (° C.) 70 63 83 DigestionTime (min.) 10 3 10 Amount of Undigested Chips 31.6 37.8 166 (g) LIGNINPRECIPITATION STEP % H₂SO₄ Solution added to black 20 20 20 liquor (v/v)Vol. of Acid Sol'n added to 130 115 300 black liquor (mL) Temp. duringAddition of Acid (° C.) 7 10 32 RESULTS Pulp recovered (g) 71 67.6 169.3Pulp recovered (%) 35.5 33.8 28.2 Lignin recovered (g) Not Weighed NotWeighed 72.5 Lignin recovered (%) Not Weighed Not Weighed 12.1 HNO₃recoverable (%) 80.2 83.2 78.8

EXAMPLE 2

[0104] This example comprised a trial that was similar to Trials 1-19 inexample 1 except that the starting lignocellulosic material was 150grams of shavings and sawdust from Hemlock, Oak and Pine tree species.Impregnation was done for 30 minutes in 15% (w/w) HNO₃ solution underheat of 50° C. The impregnated shavings and sawdust were heated for 60minutes between 50-75° C., and then for 10 minutes over 80° C. Followingthe heating step, the shavings and sawdust were contacted with analkaline solution having 1.25 moles of NaOH and 2600 mL total volume ata temperature of 52° C. for 7 minutes. Following pulp removal, ligninwas precipitated with the addition of 20% (v/v) H₂SO₄ The pulp andlignin were not weighed, but a visual inspection showed satisfactoryappearance and yield for each, proving that the method of this inventioncan be carried out with relatively small pieces of lignocellulosicmaterial such as shavings and sawdust.

EXAMPLE 3

[0105] The pulp samples produced in Trials 5, 7 and 2 from example 1were subsequently analyzed to determine the relative amounts of alphacellulose, beta cellulose and gamma cellulose therein. The results arefound in Table 6 below. TABLE 6 Trial 5 Trial 7 Trial 2 Alpha Cellulose(%) 81.3 79.6 79.2 Beta Cellulose (%) 4.4 3.7 7.5 Gamma Cellulose (%)14.3 16.7 13.3

[0106] In addition to the methods herein, the present invention alsoincludes pulp produced according to the methods herein, as well as paperproducts comprising cellulose from pulp produced according to themethods herein. Similarly, the present invention also includes ligninproduced according to the methods herein, as well as numerous otherproducts and compositions comprising lignin produced according to themethods herein, including for example, fertilizers, asphalt emulsifiers,soil stabilizers, wood products (such as plywood and particle board),oil well drilling fluids and dispersing agents for preparing concrete.

[0107] As will be apparent to those skilled in the art in the light ofthe foregoing disclosure, many alterations and modifications arepossible in the practice of this invention without departing from thescope thereof. Accordingly, the scope of the invention is to beconstrued in accordance with the substance defined by the followingclaims.

What is claimed is:
 1. A method for producing pulp and lignin fromlignocellulosic material, the pulp comprising cellulose, the methodcomprising: (a) contacting the lignocellulosic material with an aqueousacid solution to impregnate the lignocellulosic material, the aqueousacid solution comprising from about 10% to about 40% by weight of theacid; (b) heating the lignocellulosic material in two stages, the firstheating stage being carried out for a period of time which is sufficientto depolymerize lignin within the lignocellolosic material withoutsubstantially degrading the cellulose or lignin in the ligncellulosicmaterial, the second heating stage being carried out at or above theboiling point of the acid to distill off the acid; (c) contacting thelignocellulosic material with an aqueous alkaline solution under heat tosolubilize lignin in the alkaline solution, leaving a black liquor; (d)removing the pulp from the black liquor; (e) adding sufficient acid tothe black liquor to precipitate the lignin; and (f) removing the ligninfrom the liquor.
 2. A method according to claim 1 wherein the acid isnitric acid.
 3. A method according to claim 1 wherein thelignocellulosic material is contacted with the aqueous acid solution instep (a) for at least 30 minutes.
 4. A method according to claim 3wherein the lignocellulosic material is contacted with the aqueous acidsolution in step (a) from about 12 hours to about 24 hours.
 5. A methodaccording to claim 1 wherein the lignocellulosic material is contactedwith the aqueous acid solution in step (a) under heat.
 6. A methodaccording to claim 2 wherein the lignocellulosic material is contactedwith the aqueous nitric acid solution in step (a) at a temperature fromabout 50° C. to below the boiling point of the nitric acid.
 7. A methodaccording to claim 2 wherein the aqueous nitric acid solution in step(a) comprises from about 10% to about 30% by weight of the nitric acid.8. A method according to claim 7 wherein the aqueous nitric acidsolution comprises from about 15% to about 30% by weight of the nitricacid.
 9. A method according to 8 wherein the aqueous nitric acidsolution comprises from about 20% to about 25% by weight of the nitricacid.
 10. A method according to claim 2 wherein the aqueous nitric acidsolution comprises from about 10% to about 15% by weight of the nitricacid.
 11. A method according to claim 2 wherein the temperature duringthe second heating stage of step (b) is from about 73° C. to below 100°C.
 12. A method according to claim 11 wherein the temperature during thesecond heating stage of step (b) is from about 90° C. to about 95° C.13. A method according to claim 2 wherein the temperature during thefirst heating stage of step (b) is up to about 75° C.
 14. A methodaccording to claim 13 wherein the temperature during the first heatingstage of step (b) is from about 50° C. to about 75° C.
 15. A methodaccording to claim 14 wherein the temperature during the first heatingstage of step (b) is from about 60° C. to about 70° C.
 16. A methodaccording to claim 14 wherein the temperature during the first heatingstage of step (b) is from about 50° C. to about 60° C.
 17. A methodaccording to claim 14 wherein the temperature during the first heatingstage of step (b) is from about 70° C. to about 75° C.
 18. A methodaccording to claim 2 wherein the first heating stage in step (b) iscarried out for at least 15 minutes.
 19. A method according to claim 2wherein the aqueous alkaline solution comprises sodium hydroxide orpotassium hydroxide or a combination of sodium hydroxide and potassiumhydroxide.
 20. A method according to claim 2 wherein the aqueousalkaline solution comprises an amount of alkali solute which is at leastthe normal equivalent of the nitric acid in the aqueous acid solution instep (a).
 21. A method according to claim 2 wherein the aqueous alkalinesolution comprises an amount of alkali solute which is at least themolar equivalent of the nitric acid in the aqueous acid solution in step(a).
 22. A method according to claim 2 wherein the acid added in step(e) is sulfuric acid.
 23. A method according to claim 2 wherein theamount of acid added in step (e) is at least the molar amount of thealkali in the aqueous alkaline solution in step (c).
 24. A methodaccording to claim 1 wherein the black liquor is cooled from thetemperature in step (c) before the acid is added in step (e).
 25. Amethod according to claim 2 wherein the temperature of the black liquorwhen the acid is added in step (e) is up to about 75° C.
 26. A methodaccording to claim 25 wherein the temperature of the black liquor whenthe acid is added in step (e) is from about 5° C. to about 75° C.
 27. Amethod according to claim 26 wherein the temperature of the black liquorwhen the acid is added in step (e) is from about 5° C. to about 50° C.28. A method according to claim 27 wherein the temperature of the blackliquor when the acid is added in step (e) is from about 25° C. to about50° C.
 29. A method according to claim 25 wherein the temperature of theblack liquor when the acid is added in step (e) is from about 50° C. toabout 75° C.
 30. A method according to claim 1 wherein any aqueous acidsolution not absorbed by the lignocellulosic material in step (a) isremoved prior to heating the lignocellulosic material in step (b).
 31. Amethod according to claim 30 comprising collecting any aqueous acidsolution which is removed prior to step (b) and recycling the collectedaqueous acid solution for use in step (a), and comprising collecting anyacid which is distilled off in step (b) and recycling the collected acidfor use in step (a).
 32. A method according to claim 1 comprisingcontacting the lignocellulosic material with water before step (a) toincrease the moisture content in the lignocellulosic material.
 33. Amethod according to claim 1 wherein the starting moisture content of thelignocellulosic material is from about 30% to about 55% by weight ofwater.
 34. A method according to claim 1 wherein the startinglignocellulosic material comprises undigested lignocellulosic materialpreviously subjected to the method.
 35. A method according to claim 1wherein the starting lignocellulosic material comprises wood chips, woodshavings, sawdust or a combination of two or more wood chips, woodshavings and sawdust.
 36. A method according to claim 1 wherein thestarting lignocellulosic material comprises pieces of rye, wheat, hempor a combination of two or more of rye, wheat and hemp.
 37. A methodaccording to claim 1 wherein an amber liquor is left following theremoval of the lignin, and comprising processing the amber liquor afterthe lignin has been removed.
 38. A method according to claim 37 whereinthe amber liquor is processed to produce unicellular proteins oralcohols or both.
 39. A method according to claim 1 wherein any waterwhich is produced as a reaction byproduct in one or more of the steps iscollected and recycled for use in the method.
 40. A method according toclaim 2 wherein steps (a) and (c) and the first heating stage in step(b) are each carried out at a temperature from about 50° C. to about 75°C.
 41. A method according to claim 1 wherein at least one of the heatingof the impregnated lignocellulosic material in step (b), the contactingthe lignocellulosic material with the aqueous alkaline solution in step(c) and the adding the acid in step (e) is carried out with agitation.42. A method according to claim 1 wherein the heating of the impregnatedlignocellulosic material in step (b), the contacting the lignocellulosicmaterial with the aqueous alkaline solution in step (c) and the additionof the acid in step (e) are each carried out with agitation.
 43. Amethod according to claim 1 wherein the method is carried out atatmospheric pressure.
 44. A method according to claim 1 comprisingwashing, pressing, bleaching and drying the pulp removed in step (d).45. A method according to claim 1 comprising drying the lignin removedin step (f).
 46. A method according to claim 2 wherein step (c) iscarried out at a temperature up to about 75° C.
 47. A method accordingto claim 46 wherein step (c) is carried out at a temperature from about5° C. to about 75° C.
 48. A method according to claim 47 wherein step(c) is carried out at a temperature from about 50° C. to about 75° C.49. A method according to claim 46 wherein step (c) is carried out at atemperature from about 20° C. to about 50° C.
 50. A method according toclaim 47 wherein step (c) is carried out at a temperature from about 30°C. to about 40° C.
 51. A method according to claim 47 wherein step (c)is carried out at a temperature from about 40° C. to about 50° C.
 52. Amethod according to claim 1 wherein the lignocellulosic material isimmersed in the aqueous acid solution in step (a).
 53. A methodaccording to claim 1 wherein the lignocellulosic material is sprayedwith the aqueous acid solution in step (a).
 54. A method for producingpulp and lignin from lignocellulosic material, the pulp comprisingcellulose, the method comprising: (a) contacting the lignocellulosicmaterial with an aqueous nitric acid solution to impregnate thelignocellulosic material, the aqueous nitric acid solution comprisingfrom about 10% to about 40% by weight of the nitric acid, (b) heatingthe impregnated lignocellulosic material in two stages, the firstheating stage being carried out a temperature from about 50° C. to about75° C. for a period of time which is sufficient to depolymerize ligninwithin the lignocellolosic material without substantially degrading thecellulose or lignin in the ligncellulosic material, the second heatingstage being carried out at or above the boiling point of the nitric acidto distill off the nitric acid; (c) contacting the lignocellulosicmaterial with an aqueous alkaline solution at a temperature from about50° C. to about 75° C. to solubilize lignin in the alkaline solution,leaving a black liquor, the aqueous alkaline solution comprising anamount of alkali which is at least the normal amount of the nitric acidin the aqueous acid solution in step (a); (d) removing the pulp from theblack liquor; (e) cooling the black liquor and then adding an acid tothe black liquor to acidify the solution to precipitate the lignin; (f)removing the lignin, leaving an amber liquor; and (g) processing theamber liquor to produce unicellular proteins or alcohols or both,wherein any aqueous nitric acid not absorbed by the lignocellulosicmaterial in step (a) is removed and collected following step (a) priorto heating the lignocellulosic material in step (b) and then recycledfor use in step (a), and wherein any nitric acid which is distilled offis collected prior to contacting the lignocellulosic material with thealkaline solution in step (c) and then recylcled for use in step (a),and wherein the heating of the lignocellulosic material in step (b), thecontacting the lignocellulosic material with the aqueous alkalinesolution in step (c) and the addition of the acid in step (e) are eachcarried out with agitation.
 55. A method for producing pulp and lignincomprising the steps of contacting lignocellulosic material with an acidand, after removing any acid not absorbed by the lignocellulosicmaterial, heating the lignocellulosic material at a temperature up toabout 75° C. to effect the acid-catalyzed hydrolytic depolymerization ofthe lignin in the lignocellulosic material without substantiallydegrading the cellulose or lignin in the lignocellulosic material, theacid-contacting and heating steps being carried out before thelignocellulosic material is digested in an alkaline liquor, the pulpbeing removed following the digestion of the lignocellulosic material inthe alkaline liquor, the lignin being removed after being precipitatedout with the addition of an acid to the black liquor produced followingthe digestion of the lignocellulosic material in the alkaline liquor,wherein the pulp comprises cellulose.
 56. A method according to claim 55wherein the acid-contacting step comprises immersing the lignocellulosicmaterial in an aqueous solution of the acid, and wherein the acid isnitric acid and the aqueous solution comprises from about 10% to about40% by weight of nitric acid.
 57. A method according to claim 56comprising heating the lignocellulosic material after depolymerizing thelignin but before digesting the lignocellulosic material, at atemperature above the boiling point of the acid in order to distill offthe acid.
 58. A method according to claim 44 wherein any liquor that isremoved from the pulp by pressing is collected and added to the blackliquor prior to adding the acid in step (e).
 59. A method according toclaim 45 wherein any liquor which is removed from the lignin duringdrying is collected and added to the liquor after step (f), and whereinthe liquor is processed after the lignin has been precipitated andremoved.
 60. The method of claim 1 wherein enough acid is added to thesolution in step (e) to lower the pH of the solution to an acidic pH.61. A method for treating lignocellulosic material comprising: (a)contacting the lignocellulosic material with an aqueous acid solution toimpregnate the lignocellulosic material, the aqueous acid solutioncomprising from about 10% to about 40% by weight of the acid; (b)heating the lignocellulosic material in two stages, the first heatingstage being carried out for a period of time which is sufficient todepolymerize lignin within the lignocellolosic material withoutsubstantially degrading the cellulose or lignin in the ligncellulosicmaterial, the second heating stage being carried out at or above theboiling point of the acid to distill off the acid, wherein any aqueousacid solution not absorbed by the lignocellulosic material in step (a)is removed prior to heating the lignocellulosic material in step (b);(c) contacting the lignocellulosic material with an aqueous alkalinesolution under heat to solubilize lignin in the alkaline solution,leaving a black liquor; and (d) removing the pulp from the black liquor,the pulp comprising cellulose.
 62. A method according to claim 2 whereinthe amount of acid added in step (e) is at least the normal amount ofthe alkali in the aqueous alkaline solution in step (c).
 63. A methodfor producing pulp and lignin from lignocellulosic material, the pulpcomprising cellulose, the method comprising: (a) contacting thelignocellulosic material with an aqueous nitric acid solution toimpregnate the lignocellulosic material, the aqueous acid solutioncomprising from about 10% to about 40% by weight of the nitric acid; (b)heating the lignocellulosic material in two stages, the first heatingstage being carried out for a period of time which is sufficient todepolymerize lignin within the lignocellolosic material withoutsubstantially degrading the cellulose or lignin in the ligncellulosicmaterial, the second heating stage being carried out at or above theboiling point of the acid to distill off the acid, wherein any aqueousacid solution not absorbed by the lignocellulosic material in step (a)is removed prior to heating the lignocellulosic material in step (b);(c) contacting the lignocellulosic material with an aqueous alkalinesolution under heat to solubilize lignin in the alkaline solution,leaving a black liquor; (d) removing the pulp from the black liquor; (e)adding sufficient acid to the black liquor to precipitate the lignin;and (f) removing the lignin from the liquor.